Support .pyx files

[pawamoy]

Is your feature request related to a problem? Please describe.
pyx files are Cython-written modules. They're not statically supported.
When Griffe finds the source of a package that contains pyx files, these files won't be loaded.

Describe the solution you'd like
When Griffe encounters a pyx file, it could try to inspect the corresponding module.

Describe alternatives you've considered
Forcing inspection on the whole package: #94 (or hiding sources from Griffe to it falls back to inspection).
Though we lose the benefit of static analysis on the other non-Cython modules.

Additional context
mkdocstrings/mkdocstrings#603